This proposal seeks support to study the structural basis of specific macromolecular interactions typical of the cell surface. The core of the experimental approach will be X-ray diffraction studies of single crystals. The systems to be examined by this approach are (1) Neurophysin-vasopressin complex, a complex between a protein and peptide hormone; (2) delta5-3-ketosteroid isomerase, a protein that interacts specifically with a steroid hormone; (3) snake venom phosphodiesterase, a protein that recognizes specific phospholipids (crystalline); (4) Non-covalent complex between a lytic fragment of "band 3" protein (human RBC ghosts) and mammalian enzymes. In all cases but (4) crystals of these materials have been obtained.